Fractionation and Improved Enzymatic Deconstruction of Hardwoods with Alkaline Delignification

Abstract

In this work, an alkaline delignification was investigated for several industrially relevant hardwoods to understand the kinetics of xylan solubilization and degradation and the role of residual lignin content in setting cell wall recalcitrance to enzymatic hydrolysis. Between 34 and 50 % of the xylan was solubilized during the heat-up stage of the pretreatment and undergoes degradation, depolymerization, as well as substantial disappearance of the glucuronic acid substitutions on the xylan during the bulk delignification phase. An important finding is that substantial xylan is still present in the liquor without degradation. Cellulose hydrolysis yields in the range of 80 to 90 % were achievable within 24–48 h for the diverse hardwoods subjected to delignification by alkali at modest enzyme loadings. It was found that substantial delignification was not necessary to achieve these high hydrolysis yields and that hybrid poplar subjected to pretreatment removing only 46 % of the lignin was capable of reaching yields comparable to hybrid poplar pretreated to 67 or 86 % lignin removal. Decreasing the lignin content was found to increase the initial rate of cellulose hydrolysis to glucose while lignin contents under approximately 70 mg/g original biomass were found to slightly decrease the maximum extent of hydrolysis, presumably due to drying-induced cellulose aggregation and pore collapse. Pretreatments were performed on woodchips, which necessitated a “disintegration” step following pretreatment. This allowed the effect of comminution method to be investigated for the three hardwoods subjected to the highest level of delignification. It was found that additional knife-milling following distintegration did not impact either the rate or extent of glucan and xylan hydrolysis.